1. Field of the Invention
The present invention relates to a reflecting optical system and an imaging apparatus using it and, more particularly, to a reflecting optical system suitably applicable to imaging optical systems, observing optical systems, and so on in video cameras and still video cameras for forming an object image on a predetermined surface, using an optical element having a plurality of reflecting surfaces.
2. Related Background Art
A variety of imaging or observing optical systems using a refracting optical system have been proposed heretofore. These optical systems are well corrected for spherical aberration, coma, curvature of field, and so on at a reference wavelength in the visible wavelength region and also corrected similarly for various aberrations at wavelengths other than the reference wavelength. In particular, since a refracting system has so-called dispersion that refractive indices of a material such as glass differ depending upon wavelengths, imaging performance is improved by correction for chromatic aberration occurring because of the dispersion characteristics (which is so called achromatization).
For example, in the case of an optical system using a refracting lens, it is theoretically impossible to effect achromatization as well as the imaging action with a single lens. Therefore, correction for chromatic aberration is done by a combination of plural lenses different in index and dispersion.
On the other hand, there have been a variety of proposals on photographing optical systems using reflecting surfaces such as a concave mirror or a convex mirror. Since the reflecting surfaces theoretically cause no chromatic aberration, such photographing optical systems are often applied to telescopes the imaging performance of which is very susceptible to chromatic aberration.
FIG. 17 is a schematic view of a so-called mirror optical system comprised of a concave mirror and a convex mirror. In the mirror optical system of the same drawing, an object beam 104 from an object is reflected by the concave mirror 101 then travels toward the object, is converged and then is reflected by the convex mirror 102 to form an image on the image plane 103 thereafter. Reference numeral 105 is a reference axis.
This mirror optical system has the basic configuration of the so-called Cassegrainian reflecting telescope, which reduces the total length of optical system by folding an optical path of a telephoto lens system with a long total lens length comprised of refracting lenses by two opposite reflecting mirrors and which avoids chromatic aberration specific to the telephoto lens by using the mirror optical system.
In this way, in the photographing lenses with longer total lens lengths, the reflecting mirrors are conventionally used instead of the lenses to fold the optical path efficiently, thereby obtaining the mirror optical system compact and free of chromatic aberration. In a so-called catoptric optical system using only the reflecting system, it is, however, difficult to correct all aberrations occurring at the reflecting mirrors by the limited number of surfaces or in a limited space.
There are thus examples of advantageously combining the reflecting system with the refracting system to increase degrees of freedom and thereby to correct aberrations as a total system. FIG. 18 shows an example of a so-called catadioptric system as a combination of the reflecting system with the refracting system. In FIG. 18, an object beam 116 from an object is subject to refraction in refracting lenses 111, 112, thereafter is reflected by a concave mirror 113, then travels toward the object as being converged, thereafter is reflected by a convex mirror 114, and then forms an image on the image plane 115. The refracting lens system is constructed so as to correct aberration occurring at the reflecting mirrors.
However, the refracting system is the combination of the convex lens 111 with the concave lens 112 in order to suppress chromatic aberration. Although the optical path is folded efficiently by only the reflecting system to achieve the compact arrangement, it has a disadvantage of an increase of size because it requires refracting lenses with a large aperture in fact.
In addition, because of an increase in the number of components, it was necessary to assemble the respective optical components with high accuracy in order to attain necessary optional performance. In particular, because high accuracy is required for the relative position between the reflecting mirrors or for the relative position between the reflecting mirrors and the refracting lenses, adjustment of position and angle of each reflecting mirror was essential.
Proposed as a method for solving this problem is a method for forming the mirror system in a block, thereby avoiding assembling errors of the optical components caused upon assembling, for example.
Conventional examples of such elements incorporating many reflecting surfaces in a block include optical prisms such as a pentagonal roof prism or a Porro prism used in a finder system or the like, for example.
Since these prisms include a plurality of reflecting surfaces integrally formed, they are formed with high accuracy for the relation of relative position among the reflecting surfaces, which obviates a need for positional adjustment between the reflecting surfaces. In many cases, however, the principal function of these prisms is to change the traveling direction of light rays so as to invert the image and the reflecting surfaces are often flat.
In contrast with the foregoing, there are known optical systems with the reflecting surfaces of prism having curvature.
FIG. 19 is a schematic drawing of the major part of the observing optical system disclosed in the specification of U.S. Pat. No. 4,775,217. This observing optical system is an optical system for observing a view in the external field and for observing a display image displayed on an information display as overlapping the view.
In this observing optical system, a display beam 125 emitted from the display image on the information display 121 is reflected by a surface 122 to travel toward the object and then to enter a halfmirror surface 123 being a concave surface. After being reflected by this halfmirror surface 123, the display beam 125 is changed to a nearly parallel beam by refracting power of the concave surface 123, then is refracted and transmitted by the surface 122, and forms an enlarged, virtual image of the display image as entering the pupil 124 of an observer. Thus the observer can visually recognize the display image.
On the other hand, the object beam 126 from an object is incident to a surface 127 nearly parallel to the reflecting surface 122 to be refracted and then to reach the halfmirror surface 123 of concave surface. A semi-transparent film is evaporated over the concave surface 123. Thus, part of the object beam 126 passes through the concave surface 123, then is refracted and transmitted by the surface 122, and thereafter enters the observer""s pupil 124. By this, the observer visually recognizes the display image overlapping the view of the external field.
FIG. 20 is a schematic drawing of the major part of the observing optical system disclosed in Japanese Patent Application Laid-open No. 2-297516. This observing optical system is also an optical system for observing the view in the external field and for observing the display image displayed on the information display as overlapping the external view.
In this observing optical system, the display beam 134 emitted from the information display 130 passes through a flat surface 137 forming the prism Pa to enter the prism Pa and then to be incident to a parabolic reflecting surface 131. The display beam 134 is reflected by this reflecting surface 131 to become a converging beam to form an image on the focal plane 136. The display beam 134 reflected by the reflecting surface 131 at this time is totally reflected between the two parallel flat surfaces 137 and 138 constituting the prism Pa and then reaches the focal plane 136, thereby achieving reduction of the thickness of the total optical system.
Then the display beam 134 emerging as diverging light from the focal plane 136 is totally reflected between the flat surface 137 and the flat surface 138 and then is incident to a halfmirror 132 of a parabolic surface. The display beam is reflected by the halfmirror surface 132 to form an enlarged, virtual image of the display image by the refracting power thereof and to become a nearly parallel beam. The parallel beam passes through the surface 137 to enter the observer""s pupil 133, thereby permitting the observer to recognize the display image.
On the other hand, the object beam 135 from the external field passes through a surface 138b forming the prism Pb and then passes through the halfmirror 132 of the parabolic surface. Then the object beam 135 passes through the surface 137 to enter the observer""s pupil 133. The observer visually recognizes the display image overlapping the view of the external field. Further, there are examples applying an optical element to the reflecting surface of prism, for example, the optical heads for optical pickup disclosed in Japanese Patent Application Laid-open No. 5-12704, No. 6-139612, and so on. These are arranged so that light from a semiconductor laser is reflected by a Fresnel surface or a hologram surface and thereafter is focused on a disk surface and that reflected light from the disk is guided to a detector.
In the optical systems with many reflecting surfaces formed in a block as described above, however, aberration correction is not done by positively constructing the catadioptric system in a block and they thus have a problem of chromatic aberration occurring at the incident and emergent surfaces because the block is made of a medium of glass or the like having the dispersion characteristics, in particular.
The prime object of the both observing optical systems disclosed in the specification of U.S. Pat. No. 4,775,217 and in the bulletin of Japanese Patent Application Laid-open No. 2-297516 as described above is the pupil imaging action and change of traveling direction of ray for efficiently transmitting the display image displayed on the information display located away from the observer""s pupil thereto, but they directly disclose nothing about the technology for positively correcting aberration by the reflecting surface with curvature.
In addition, aberration correction of the entire system is not made by positively combining the reflecting system with the refracting system, and especially, nothing is directly disclosed as to the technology for correction for chromatic aberration occurring at the incident and emergent surfaces.
Further, the optical systems for optical pickup disclosed in the bulletin of Japanese Patent Application Laid-open No. 5-12704, the bulletin of Japanese Patent Application Laid-open No. 6-139612, and so on are limited to applications to a detecting optical system, and especially, they do not satisfy the imaging performance for the imaging apparatus using an area type image pickup device such as a CCD. Further, the operation wavelength band is extremely narrow and chromatic aberration is not corrected for over the visible light region, different from the photographing optical system.
An object of the present invention is to provide a reflecting optical system arranged in such a manner that, in picking up an image using an optical element obtained by integrally forming a plurality of internal reflecting surfaces of curved or flat surfaces in a transparent body, the various aberrations of the total system is well corrected for by properly setting curvature of the incident surface or/and emergent surface of the optical element and the object position or/and image position thereof, especially chromatic aberration is corrected for at a high level, thereby improving the imaging performance, and also to provide an imaging apparatus using the reflecting optical system.
The reflecting optical system of the present invention is:
(1-1) a reflecting optical system arranged in such a manner that a beam from an object is incident into an incident surface formed in a surface of a transparent body, then is reflected by a reflecting surface of internal reflection comprised of a curved surface provided in a part of the transparent body, and thereafter emerges from an emergent surface of the transparent body,
which is characterized in that a radius of curvature of the incident surface is set to be nearly equal to a distance from a vertex of the incident surface to the object on a reference axis.
Especially, the reflecting optical system is characterized in that:
(1-1-1) the reflecting surface is decentered relative to the incident surface;
(1-1-2) the beam forms an intermediate image inside the transparent body;
(1-1-3) the center of curvature of the incident surface is set on the object side with respect to the incident surface;
(1-1-4) the aforementioned object is an image formed on the light exit side of the incident surface by another optical system and the incident surface is a convex surface; or
(1-1-5) the aforementioned object is an image formed on the light entrance side of the incident surface by another optical system and the incident surface is a concave surface.
Further, the reflecting optical system of the present invention is:
(1-2) a reflecting optical system arranged in such a manner that a beam from an object is incident into an incident surface formed in a surface of a transparent body, then is reflected by a reflecting surface of internal reflection comprised of a curved surface provided in a part of the transparent body, and thereafter emerges from an emergent surface of the transparent body to form an image,
which is characterized in that a radius of curvature of the emergent surface is set to be nearly equal to a distance from a vertex of the emergent surface to the image on a reference axis.
Especially, the reflecting optical system is characterized in that:
(1-2-1) the reflecting surface is decentered relative to the incident surface;
(1-2-2) the beam forms an intermediate image inside the transparent body;
(1-2-3) the center of curvature of the emergent surface is set on the image side with respect to the emergent surface;
(1-2-4) the emergent surface is a concave surface and the aforementioned image is formed on the light exit side of the emergent surface; or
(1-2-5) the emergent surface is a convex surface and the aforementioned image is formed on the light entrance side of the emergent surface.